1. Field of the Invention
The present invention relates to a method for smelting reduction of iron ore, and particularly to a method wherein carbonaceous material is used as both a fuel and a reducing agent for the iron ore which is being smelted and reduced in a basic oxygen furnace and an apparatus therefor.
2. Description of the Related Arts
Smelting reduction method has recently been developed as a substitution for a blast furnace iron-making method, to overcome some disadvantages in that the blast furnace iron-making method requires not only an expensive construction cost but also a vast construction site.
In a prior art smelting reduction method, iron ore is prereduced by means of an exhaust gas, and then the prereduced iron ore is charged together with carbonaceous material and flux into a smelting reduction furnace. Furthermore, oxygen gas and stirring gas are blown into the smelting reduction furnace. Thus, the carbonaceous materials are consumed thanks to the precharged molten metal and, at the same time, C contained in the carbonaceous material is oxidized by the oxygen gas. Through the heat produced by this oxidation, the ore is smelted and reduced by means of C contained in the carbonaceous materials. CO gas generating in the molten metal is post-combusted, by in excess of the oxygen gas, into CO.sub.2 gas. The sensible heat of this CO.sub.2 gas is transferred to slag and iron particles in the slag covering the surface of the molten metal, and then, to the molten metal.
In this way, the iron ore is reduced to the molten metal. In this process, in order to lighten the load of reduction process in a smelting reduction furnace, iron ore is prereduced, for example, at a reduction degree of 60 to 75% before the iron ore is charged into the smelting reduction furnace as disclosed in Japanese Examined Patent Publication No. 43406/86. Resultantly, an exhaust gas from the smelting reduction furnace becomes a with a low degree of oxidation gas which is highly reducing and a large amount of the exhaust gas is required.
If iron ore before being charged into the smelting reduction furnace is prereduced at a ratio of at least 30% for the purpose of the lightening the load of reduction process of smelting reduction furnace, the oxidation degree (hereinafter referred to as "OD") of the exhaust gas from the smelting reduction furnace, where the OD is represented by the formula of "(H.sub.2 O+CO.sub.2)/(H.sub.2 +H.sub.2 O+CO+CO.sub.2)" is required to be lowered. Resultantly the amount of the exhaust gas is necessarily increased, as shown, for example, in the Japanese Examined Patent Publication No. 43406/86. This increase of the exhaust gas, naturally, increases the production cost. Consequently, to obtain the high prereduction ratio of the iron ore, exhaust gas having a low OD as explained above is required, and still the retention time of the iron ore in the prereduction furnace has to become longer than that in the smelting reduction furnace. Therefore, it becomes difficult to control the balancing of cycles of charge of the prereduced iron ore and discharge of the produced molten metal. This necessarily results in restricting control range strictly in the smelting reduction furnace.
Furthermore, in order to raise the melting speed of iron ore and to obtain the speed-up of reduction of the iron ore, a method of post-combusting CO gas in the smelting reduction furnace and making use of the heat generated therefrom has conventionally been employed, wherein O.sub.2 gas for post-combustion is introduced through tuyeres placed at upper wall of the smelting reduction furnace. However, in the conventional method, although the temperature of the exhaust gas can be elevated when the ratio of the post combustion is raised, the sensible heat transfer to the molten metal is not sufficient. This results in being forced to discharge high temperature exhaust gas. This method has a difficulty in that such a high temperature gas will attack the inner refractory wall of the smelting reduction furnace.
Therefore, it has been hitherto generally conceived that the OD of the exhaust gas cannot be raised so much.